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In [[transcendence theory]], a mathematical discipline, '''Baker's theorem''' gives a lower bound for linear combinations of [[logarithm]]s of [[algebraic number]]s. The result, proved by {{harvs|txt|authorlink=Alan Baker (mathematician)|first=Alan|last= Baker|year1=1966|year2=1967a|year3=1967b}}, subsumed many earlier results in transcendental number theory and solved a problem posed by [[Alexander Gelfond]] nearly fifteen years earlier.<ref>See the final paragraph of Gelfond (1952).</ref>
Baker used this to prove the transcendence of many numbers, and to derive effective bounds for the solutions of some Diophantine equations, and to solve the [[class number problem]] of finding all imaginary [[quadratic field]]s with [[Class numbers of imaginary quadratic fields|class number]] 1.

==History==
To simplify notation, we introduce the set ''L'' of logarithms of nonzero algebraic numbers, that is
:<math>L= \left \{\lambda\in\mathbf{C} : \ e^\lambda\in\overline{\mathbf{Q}}^\times \right \}.</math>
Using this notation, several results in transcendental number theory become much easier to state. For example the [[Hermite–Lindemann theorem]] becomes the statement that any nonzero element of ''L'' is transcendental.

In 1934, Alexander Gelfond and [[Theodor Schneider]] independently proved the [[Gelfond–Schneider theorem]]. This result is usually stated as: if ''a'' is algebraic and not equal to 0 or 1, and if ''b'' is algebraic and irrational, then ''a''''b'' is transcendental. Equivalently, though, it says that if λ1 and λ2 are elements of ''L'' that are linearly independent over the rational numbers, then they are linearly independent over the algebraic numbers. So if λ1 and λ2 are elements of ''L'' and λ2 isn't zero, then the quotient λ1/λ2 is either a rational number or transcendental. It can't be an algebraic irrational number like √2.

Although proving this result of "rational linear independence implies algebraic linear independence" for two elements of ''L'' was sufficient for his and Schneider's result, Gelfond felt that it was crucial to extend this result to arbitrarily many elements of ''L''. Indeed, from {{harvtxt|Gelfond|1952|p= 177}}:
{{Quotation|…one may assume … that the most pressing problem in the theory of transcendental numbers is the investigation of the measures of transcendence of finite sets of logarithms of algebraic numbers.}}

This problem was solved fourteen years later by Alan Baker and has since had numerous applications not only to transcendence theory but in [[algebraic number theory]] and the study of [[Diophantine equations]] as well. Baker received the [[Fields medal]] in 1970 for both this work and his applications of it to Diophantine equations.

==Statement==
With the above notation, Baker's theorem is a nonhomogeneous generalisation of the Gelfond–Schneider theorem. Specifically it states:

<blockquote>'''Baker's Theorem.''' If λ1,…,λ''n'' are elements of ''L'' that are linearly independent over the rational numbers, then for any algebraic numbers β0, ..., β''n'', not all zero, we have
:<math>|\beta_0+\beta_1\lambda_1+\cdots+\beta_n\lambda_n|>H^{-C}</math>
where ''H'' is the maximum of the [[height function|heights]] of the β's and ''C'' is an [[Effective results in number theory|effectively computable]] number depending on ''n'', the λ's, and the maximum ''d'' of the degrees of the β's. (If β0 is nonzero then the assumption that the λ's are linearly independent can be dropped.) In particular this number is nonzero, so 1 and the λ's are linearly independent over the algebraic numbers.</blockquote>

Just as the Gelfond–Schneider theorem is equivalent to the statement about the transcendence of numbers of the form ''a''''b'', so too Baker's theorem implies the transcendence of numbers of the form
:<math>a_1^{b_1}\cdots a_n^{b_n},</math>
where the ''b''''i'' are all algebraic, irrational, and 1, ''b''1,…,''b''''n'' are linearly independent over the rationals, and the ''a''''i'' are all algebraic and not 0 or 1.

{{harvtxt|Baker|1977}} also gave several versions with explicit constants. For example, if eλ''j'' = α''j'' has height at most ''A''''j'' ≥ 4 and all the numbers β''j'' have height at most ''B'' ≥ 4 then the linear form
:<math>\Lambda=\beta_0+\beta_1\lambda_1+\cdots+\beta_n\lambda_n</math>
is either 0 or satisfies
:<math> \log|\Lambda|>(16nd)^{200n}\Omega(\log\Omega-\log\log A_n)(\log B+\log\Omega)</math>
where
:<math>\Omega=\log A_1 \log A_2 \cdots \log A_n</math>
and the field generated by all the α's and β's over the rationals has degree at most ''d''. In the special case when β0=0 and all the β''j'' are rational integers, the rightmost term log Ω can be deleted.

An explicit result by Baker and [[Gisbert Wüstholz|Wüstholz]] for a linear form Λ with integer coefficients yields a lower bound of the form

:<math>\log\vert\Lambda\vert>-C h(\alpha_1)h(\alpha_2)\cdots h(\alpha_n) \log \left (\max\{\vert\beta_1\vert,\ldots,\vert\beta_n\vert\} \right ),</math>

with a constant ''C''

:<math>C = 18(n + 1)!\cdot n^{n+1}\cdot (32d)^{n+2}\log(2nd),</math>

where ''d'' is the degree of the [[number field]] generated by the α''i''.

==Baker's method==
Baker's proof of his theorem is an extension of the argument given by {{harvtxt|Gel'fond|1960|loc=chapter III, section 4}}.
The main ideas of the proof are illustrated by the proof of the following qualitative version of the theorem of {{harvtxt|Baker|1966}} described by {{harvtxt|Serre|1971}}: if the numbers 2π''i'' and log ''a''1,..., log ''a''''n'' are linearly independent over the rational numbers, for nonzero algebraic numbers ''a''1,..., ''a''''n'', then they are linearly independent over the algebraic numbers. The precise quantitative version of Bakers theory can be proved by replacing the conditions that things are zero by conditions that things are sufficiently small throughout the proof.

The main idea of Bakers proof is to construct an [[auxiliary function]] Φ(''z''1,...,''z''''n''−1) of several variables that vanishes to high order at many points of the form Φ(l,l,...,l), then repeatedly show that it vanishes to lower order at even more points of this form. Finally the fact that it vanishes (to order 1) at enough points of this form implies using [[Vandermonde determinant]]s that there is a multiplicative relation between the numbers ''a''''i''.

===Construction of the function Φ===
Assume there is a relation
:<math>\beta_1\log \alpha_1+\cdots+\beta_{n-1}\log\alpha_{n-1}=\log \alpha_n</math>
for algebraic numbers α1, ..., α''n'', β1, ..., β''n''−1 The function Φ is of the form
:<math>\Phi(z_1,\ldots,z_{n-1}) = \sum_{\lambda_1=0}^L\cdots \sum_{\lambda_n=0}^L p(\lambda_1\ldots,\lambda_n) \alpha_1^{(\lambda_1+\lambda_n\beta_1)z_1} \cdots\alpha_{n-1}^{(\lambda_{n-1}+\lambda_n\beta_{n-1})z_{n-1}}</math>
The integer coefficients ''p'' are chosen so that they are not all zero and Φ and its derivatives of order at most some constant ''M'' vanish at ''z''1= ... =''z''''n''−1 = ''l'', for integers ''l'' with 0≤''l''≤''h'' for some constant ''h''. This is possible because these conditions are homogeneous linear equations in the coefficients ''p'', which have a non-zero solution provided the number of unknown variables ''p'' is larger than the number of equations. The linear relation between the logs of the α's is needed to cut down the number of linear equations that have to be satisfied. Moreover, using [[Siegel's lemma]], the sizes of the coefficients ''p'' can be chosen to be not too large. The constants ''L'', ''h'', and ''M'' have to be carefully adjusted to that the next part of the proof works, and are subject to some constraints, which are roughly:
*''L'' must be somewhat smaller than ''M'' to make the argument about extra zeros below work.
*A small power of ''h'' must be larger than ''L'' to make the final step of the proof work.
*''L''''n'' must be larger than about ''M''''n−1''''h'' in order that it is possible to solve for the coefficients ''p''.
The constraints can be satisfied by taking ''h'' to be sufficiently large, ''M'' to be some fixed power of ''h'', and ''L'' to be a slightly smaller power of ''h''. Baker took ''M'' to be about ''h''2 and ''L'' to be about ''h''2−1/2''n''.

The linear relation between the logarithms of the α's is used to reduce ''L'' slightly; roughly speaking, without it the condition ''L''''n'' must be larger than about ''M''''n−1''''h'' would become ''L''''n'' must be larger than about ''M''''n''''h'', which is incompatible with the condition that ''L'' is somewhat smaller than ''M''.

===Zeros of Φ(l, ..., l)===
The next step is to show that Φ vanishes to slightly smaller order at many more points of the form ''z''1 = ... = ''z''''n''−1 =''l'' for integers ''l''. This idea was Baker's key innovation: previous work on this problem involved trying to increase the number of derivatives that vanish while keeping the number of points fixed, which does not seem to work in the multivariable case. This is done by combining two ideas; First one shows that the derivatives at these points are quite small, by using the fact that many derivatives of Φ vanish at many nearby points. Then one shows that derivatives of Φ at this point are given by algebraic integers times known constants. If an algebraic integer has all its conjugates bounded by a known constant, then it cannot be too small unless it is zero, because the product of all conjugates of a nonzero algebraic integer is at least 1 in absolute value. Combining these two ideas implies that Φ vanishes to slightly smaller order at many more points ''z''1 = ... = ''z''''n''−1 =''l''. This part of the argument requires that Φ does not increase too rapidly; the growth of Φ depends on the size of ''L'', so requires a bound on the size of ''L'', which turns out to be roughly that ''L'' must be somewhat smaller than ''M''. More precisely, Baker showed that since Φ vanishes to order ''M'' at ''h'' consecutive integers, it also vanishes to order ''M''/2 at ''h''1+1/8''n'' consecutive integers 1, 2, 3, .... Repeating this argument ''J'' times shows that Φ vanishes to order ''M''/2''J'' at ''h''1+''J''/8''n'' points, provided that ''h'' is sufficiently large and ''L'' is somewhat smaller than ''M''/2''J''.

One then takes ''J'' large enough that ''h''1+''J''/8''n'' > (''L''+1)''n'' (''J'' larger than about 16''n'' will do if ''h''2 > ''L'') so that that Φ(''l'', ..., ''l'') = 0 for all integers ''l'' with 1 ≤ ''l'' ≤ (''L''+1)''n''.

===Completion of the proof===
The condition that Φ(''l'', ..., ''l'')=0 for all integers ''l'' with 1≤ ''l''≤ (''L''+1)''n'' can be written as
:<math> \sum_{\lambda_1=0}^L\cdots \sum_{\lambda_n=0}^Lp(\lambda_1\ldots,\lambda_n)\alpha_1^{\lambda_1l}\cdots\alpha_{n}^{\lambda_{n}l} = 0</math>
This consists of (''L''+1)''n'' homogeneous linear equations in the (''L''+1)''n'' unknowns ''p'', and by assumption has a non-zero solution ''p'', so the determinant of the matrix of coefficients must vanish. However this matrix is a [[Vandermonde matrix]], so the formula for the determinant of such a matrix forces an equality two of the values
:<math> \alpha_1^{\lambda_1}\cdots\alpha_{n}^{\lambda_{n}}</math>
so the numbers α1,...,α''n'' are multiplicatively dependent. Taking logs then shows that 2π''i'', log α1,...,log α''n'' are linearly dependent over the rationals.

===Extensions and generalizations===
{{harvtxt|Baker|1966}} in fact gave a quantitative version of the theorem, giving effective lower bounds for the linear form in logarithms. This is done by a similar argument, except statements about something being zero are replaced by statements giving a small upper bound for it, and so on.

{{harvtxt|Baker|1967a}} showed how to eliminate the assumption about 2π''i'' in the theorem. This requires a modification of the final step of the proof. One shows that many derivatives of the function φ(''z'') = Φ(''z'', ..., ''z'') vanish at ''z''=0, by an argument similar to the one above. But these equations for the first (''L''+1)''n'' derivatives again give a homogeneous set of linear equations for the coefficients ''p'', so the determinant is zero, and is again a Vandermonde determinant, this time for the numbers λ1log α1 + ... + λnlog αn. So two of these expressions must be the same which shows that log α1,...,log α''n'' are linearly dependent over the rationals.

{{harvtxt|Baker|1967b}} gave an inhomogeneous version of the theorem, showing that β0+β1log(α1) + ... + β''n'' log(α''n'') is nonzero for nonzero algebraic numbers β0, ..., β''n'', α1, ..., α''n'', and moreover giving an effective lower bound for it. The proof is similar to the homogeneous case: one can assume that
:<math>\beta_0+\beta_1\log \alpha_1+\cdots+\beta_{n-1}\log\alpha_{n-1}=\log \alpha_n</math>
and one inserts an extra variable ''z''0 into Φ as follows:
:<math>\Phi(z_0,\ldots,z_{n-1}) = \sum_{\lambda_0=0}^L\cdots \sum_{\lambda_n=0}^Lp(\lambda_0\ldots,\lambda_n)z_0^{\lambda_0}e^{\lambda_n\beta_0z_0}\alpha_1^{(\lambda_1+\lambda_n\beta_1)z_1}\cdots\alpha_{n-1}^{(\lambda_{n-1}+\lambda_n\beta_{n-1})z_{n-1}}</math>

==Corollaries==
As mentioned above, the theorem includes numerous earlier transcendence results concerning the exponential function, such as the Hermite–Lindemann theorem and Gelfond–Schneider theorem. It is not quite as encompassing as the still unproven [[Schanuel's conjecture]], and does not imply the [[six exponentials theorem]] nor, clearly, the still open [[four exponentials conjecture]].

The main reason Gelfond desired an extension of his result was not just for a slew of new transcendental numbers. In 1935 he used the tools he had developed to prove the [[Gelfond–Schneider theorem]] to derive a lower bound for the quantity
:<math>|\beta_1\lambda_1+\beta_2\lambda_2|</math>
where β1 and β2 are algebraic and λ1 and λ2 are in ''L''.<ref>See Gelfond (1952) and Sprindžuk (1993) for details.</ref> Baker's proof gave lower bounds for quantities like the above but with arbitrarily many terms, and he could use these bounds to develop effective means of tackling Diophantine equations and to solve Gauss' [[class number problem]].

==Extensions==
Baker's theorem grants us the linear independence over the algebraic numbers of logarithms of algebraic numbers. This is weaker than proving their [[Algebraic independence|''algebraic'' independence]]. So far no progress has been made on this problem at all. It has been conjectured<ref>Waldschmidt, conjecture 1.15.</ref> that if λ1,…,λ''n'' are elements of ''L'' that are linearly independent over the rational numbers, then they are algebraically independent too. This is a special case of Schanuel's conjecture, but so far it remains to be proved that there even exist two algebraic numbers whose logarithms are algebraically independent. Indeed, Baker's theorem rules out linear relations between logarithms of algebraic numbers unless there are trivial reasons for them; the next most simple case, that of ruling out [[Homogeneous polynomial|homogeneous]] quadratic relations, is the still open [[four exponentials conjecture]].

Similarly, extending the result to algebraic independence but in the [[p-adic]] setting, and using the [[P-adic exponential function|''p''-adic logarithm function]], remains an open problem. It is known that proving algebraic independence of linearly independent ''p''-adic logarithms of algebraic ''p''-adic numbers would prove [[Leopoldt's conjecture]] on the ''p''-adic ranks of units of a number field.

==See also==
* [[Analytic subgroup theorem]]

==Notes==
{{Reflist}}

==References==
*{{Citation | last1=Baker | first1=Alan | title=Linear forms in the logarithms of algebraic numbers. I | doi=10.1112/S0025579300003971 | mr=0220680 | year=1966 | journal=Mathematika. A Journal of Pure and Applied Mathematics | issn=0025-5793 | volume=13 | pages=204–216}}
*{{Citation | last1=Baker | first1=Alan | title=Linear forms in the logarithms of algebraic numbers. II | doi=10.1112/S0025579300008068 | mr=0220680 | year=1967a | journal=Mathematika. A Journal of Pure and Applied Mathematics | issn=0025-5793 | volume=14 | pages=102–107}}
*{{Citation | last1=Baker | first1=Alan | title=Linear forms in the logarithms of algebraic numbers. III | doi=10.1112/S0025579300003843 | mr=0220680 | year=1967b | journal=Mathematika. A Journal of Pure and Applied Mathematics | issn=0025-5793 | volume=14 | pages=220–228}}
*{{Citation | last1=Baker | first1=Alan | title=Transcendental number theory | url=http://books.google.com/books?isbn=052139791X | publisher=[[Cambridge University Press]] | edition=2nd | series=Cambridge Mathematical Library | isbn=978-0-521-39791-9 | mr=0422171 | year=1990}}
*{{Citation | last1=Baker | first1=Alan | title=Transcendence theory: advances and applications (Proc. Conf., Univ. Cambridge, Cambridge, 1976) | publisher=[[Academic Press]] | location=Boston, MA | isbn=978-0-12-074350-6 | mr=0498417 | year=1977 | chapter=The theory of linear forms in logarithms | pages=1–27}}
*{{Citation | last1 = Baker | first1 = A. | author1-link = Alan Baker (mathematician) | last2 = Wüstholz | first2 = G. | author2-link = Gisbert Wüstholz | doi = 10.1515/crll.1993.442.19 | journal = [[Crelle's Journal|Journal für die Reine und Angewandte Mathematik]] | mr = 1234835 | pages = 19–62 | title = Logarithmic forms and group varieties | volume = 442 | year = 1993}}.
*{{Citation | last1=Baker | first1=Alan | last2=Wüstholz | first2=G. | title=Logarithmic forms and Diophantine geometry | url=http://books.google.com/books?isbn=978-0-521-88268-2 | publisher=[[Cambridge University Press]] | series=New Mathematical Monographs | isbn=978-0-521-88268-2 | mr=2382891 | year=2007 | volume=9}}
*{{Citation | last1=Gel'fond | first1=A. O. | title=Transcendental and algebraic numbers | origyear=1952 | url=http://books.google.com/books?isbn=0486495264 | publisher=[[Dover Publications]] | location=New York | series=Dover Phoenix editions | isbn=978-0-486-49526-2 | mr=0057921 | year=1960}}
*{{Citation | last1=Serre | first1=Jean-Pierre | author1-link=Jean-Pierre Serre | title=Séminaire Bourbaki. Vol. 1969/70: Exposés 364--381 | origyear=1969 | url=http://www.numdam.org/item?id=SB_1969-1970__12__73_0 | publisher=[[Springer-Verlag]] | location=Berlin, New York | series=Lecture Notes in Mathematics | year=1971 | volume=180 | chapter=Travaux de Baker (Exposé 368) | pages=73–86}}
*{{Citation | last1=Sprindžuk | first1=Vladimir G. | title=Classical Diophantine equations | publisher=[[Springer-Verlag]] | location=Berlin, New York | series=Lecture Notes in Mathematics | isbn=978-3-540-57359-3 | doi=10.1007/BFb0073786 | mr=1288309 | year=1993 | volume=1559}}
*{{Citation | last1=Waldschmidt | first1=Michel | title=Diophantine approximation on linear algebraic groups | url=http://books.google.com/books?isbn=3-540-66785-7 | publisher=[[Springer-Verlag]] | location=Berlin, New York | series=Grundlehren der Mathematischen Wissenschaften | isbn=978-3-540-66785-8 | mr=1756786 | year=2000 | volume=326}}

[[Category:Transcendental numbers]]
[[Category:Theorems in number theory]]

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91.202.129.190 at 18:52, 9 October 2011